The present invention relates to a plastic optical fiber and a plastic optical fiber cable, characterized by a high transmission power and a wide bandwidth and well suited for optical communication media, as well as to an optical transmission device.
In the field of short-distance communication such as intra-equipment, inter-equipment communication for cars or audio equipments, step index (SI) type plastic optical fibers (POFs) are currently being used as information transmission media. In recent years, with increase in information content for transmission, there have been growing demands that the SI type POF should have a still wider bandwidth.
To meet this requirement of a wider bandwidth, multi-layered structure in which a plurality of core layers with different refractive indices is concentrically formed has been adopted for the core therein.
In Japanese Patent Application Laid-open No. 160956/1998, it is disclosed that, even if, for the sake of easiness in fabrication, the number of layers in the core is kept mere two and the core is made only double-layered, this structure can provide a POF with a low loss transmission characteristic as well as a wide bandwidth by setting each core diameter and refractive index difference between each core layer and the clad to satisfy specific conditions. These specific conditions are therein described as follows. In a plastic optical fiber having a clad, a first core disposed inside the clad and a second core disposed inside the first core, the ratio (X=D2/D1) of the second core diameter (D2) to the first core diameter (D1) is 0.3xe2x89xa6Xxe2x89xa60.95, and the ratio (Y=xcex942/xcex941) of the refractive index difference (xcex942) between the first core and the clad to the refractive index difference (xcex941) between the second core and the clad is 0.4xe2x89xa6Y xe2x89xa60.95, and besides the sum of afore-mentioned two ratios (X+Y) is X+Yxe2x89xa61.9. With respect to the refractive index difference (xcex941) between the second core and the clad, it is also described that its lower limit is equal to or greater than 0.01 and its upper limit, equal to or less than 0. 1.
Meanwhile, for optical communication such as a link, it is essential to transmit a light with a high transmission power therethrough to an optical receiver. Yet, the optical fiber disclosed in Japanese Patent Application Laid-open No. 160956/1998 fails to transmit the light with a sufficiently high power, due to inappropriate setting of layer thickness and refractive indices for respective layers therein. Further, the optical fiber is manufactured by a method comprising steps of forming an outer circular layer and thereafter introducing the material of an inner circular layer inside that outer circular layer, and, while rotating these, carrying out polymerization of the material of the inner circular layer so that a layer (a compatibly blended layer) made of a polymer blend is each formed to a thickness almost exceeding 15 xcexcm between two core layers and between a core layer and the clad. When these compatibly blended layers are formed as thickly as described above, the light scattering loss becomes considerably high, causing a problem of large transmission loss. Moreover, since the light scattering loss produced in the compatibly blended layer affects the transmission bandwidth, there are occasions that thinning of the compatibly blended layer in this optical fiber results in lowering of the transmission bandwidth. Further, in an optical transmission device utilizing this optical fiber, if the excitation numerical aperture (NA) of the light source is made larger so as to increase the quantity of incident light coupled into the optical fiber and heighten the transmission power, the transmission bandwidth of the optical fiber becomes narrower, which may lead to a problem that signals cannot be transmitted at high speed.
To examine this point further, the transmission bandwidth and transmission power for a POF manufactured by the same rotational polymerization method as described in Japanese Patent Application Laid-open No. 160956/1998 were compared with those for a POF manufactured by the continuous conjugate spinning method. In both cases of those POFs, the outer diameter and the clad thickness were set to be 750 xcexcm and 10 xcexcm, respectively, and X=0.837 and Y=0.7. The observation of the cross-sections of those POFs under a microscope, however, revealed that, while, in the POF manufactured by the rotational polymerization method, there was formed a compatibly blended layer with a thickness of 21 xcexcm around the interface between layers of the first core and the second core, in the POF manufactured by the continuous conjugate spinning method, there was found no compatibly blended layer. When the transmission bandwidths for these POFs were measured in 50 m transmission under the condition of all mode excitation, the bandwidth for the POF manufactured by the rotational polymerization method was 710 MHz, although the bandwidth for the POF manufactured by the continuous conjugate spinning method was 344 MHz. Further, when a SI type POF with a single-layered core in which, as the core material and the clad material, the inner core layer material and the clad material of the afore-mentioned POF were utilized, respectively, and the outer diameter and the clad thickness thereof were set to be 750 xcexcm and 10 xcexcm, respectively, was used as the reference POF, that is, the transmission power of this reference POF was taken to be 1, the relative transmission power of the afore-mentioned POFs having a first core and a second core was measured, using a light source with an excitation NA of 0.6. Thereat, the transmission power of the POF manufactured by the rotational polymerization method was 0.65. In contrast with this, the transmission power of the POF manufactured by the continuous conjugate spinning method was 0.81.
An object of the present invention is to provide a plastic optical fiber and a plastic optical fiber cable, with a high transmission power and a wide bandwidth. Another object of the present invention is to provide an optical transmission device which can transmit signals at high speed.
The present invention relates to a plastic optical fiber having a first core, a second core with a different refractive index from the first core, disposed concentrically on the outside of the first core, and a clad disposed concentrically on the outside of the second core, wherein:
a ratio X (r1/r2) of a first core radius r1 to a second core radius r2 and a ratio Y (n2/n1) of a refractive index difference n2 between the second core and the clad to a refractive index difference n1, between the first core and the clad are in a range where the following Expressions
Yxe2x89xa7xe2x88x921.134X+1.0518xe2x80x83xe2x80x83(1) 
Xxe2x89xa6xe2x88x921.4842Y2+1.1097Y+0.7097xe2x80x83xe2x80x83(2) 
are satisfied, and besides
Y less than 0.4 or Xxe2x89xa70.76 
is satisfied (only excepting that X=0.8 and Y=0.35 as well as X=0.8 and Y=0.45).
Further, the present invention relates to a plastic optical fiber having a first core, a second core with a different refractive index from the first core, disposed concentrically on the outside of the first core, and a clad disposed concentrically on the outside of the second core, wherein:
a ratio X (r1/r2) of a first core radius r1 to a second core radius r2 and a ratio Y (n2/n1) of a refractive index difference n2 between the second core and the clad to a refractive index difference n1, between the first core and the clad are in a range where the following Expressions
Yxe2x89xa7xe2x88x921.134X+1.0518xe2x80x83xe2x80x83(1) 
Xxe2x89xa6xe2x88x921.4842Y2+1.1097 Y+0.7097xe2x80x83xe2x80x83(2) 
Yxe2x89xa60.7xe2x80x83xe2x80x83(3) 
Xxe2x89xa70.5xe2x80x83xe2x80x83(4) 
are satisfied (only excepting that X=0.8 and Y=0.35; X=0.8 and Y=0.45 as well as X=0.6 and Y=13/22).
Further, the present invention relates to a plastic optical fiber having a first core, a second core with a different refractive index from the first core, disposed concentrically on the outside of the first core, and a clad disposed concentrically on the outside of the second core, wherein:
a ratio X (r1/r2) of a first core radius r1 to a second core radius r2 and a ratio Y (n2/n1) of a refractive index difference n2 between the second core and the clad to a refractive index difference n1, between the first core and the clad are in a range where the following Expressions
Yxe2x89xa6xe2x88x923.333X+3.1xe2x80x83xe2x80x83(5) 
xe2x80x83Yxe2x89xa72.5Xxe2x88x921.625xe2x80x83xe2x80x83(6)
Yxe2x89xa7xe2x88x920.833X+0.975xe2x80x83xe2x80x83(7) 
Yxe2x89xa7xe2x88x921.429X+1.386xe2x80x83xe2x80x83(8) 
Yxe2x89xa60.5xe2x80x83xe2x80x83(9) 
Yxe2x89xa70.35xe2x80x83xe2x80x83(10) 
are satisfied.
Further, the present invention relates to a plastic optical fiber having a first core, a second core with a different refractive index from the first core, disposed concentrically on the outside of the first core, and a clad disposed concentrically on the outside of the second core, wherein:
a ratio X (r1/r2) of a first core radius r1 to a second core radius r2 and a ratio Y (n2/n1) of a refractive index difference n2 between the second core and the clad to a refractive index difference n1 between the first core and the clad are in a range where the following Expressions
Yxe2x89xa7xe2x88x921.429X+1.429xe2x80x83xe2x80x83(11) 
Yxe2x89xa6xe2x88x922.5X+2.4xe2x80x83xe2x80x83(12) 
Yxe2x89xa60.5xe2x80x83xe2x80x83(13) 
Yxe2x89xa70.4xe2x80x83xe2x80x83(14) 
are satisfied.
Further, the present invention relates to a plastic optical fiber having a first core, a second core with a different refractive index from the first core, disposed concentrically on the outside of the first core, and a clad disposed concentrically on the outside of the second core, wherein:
a ratio X (r1/r2) of a first core radius r1, to a second core radius r2 and a ratio Y (n2/n1) of a refractive index difference n2 between the second core and the clad to a refractive index difference n2, between the first core and the clad are in a range where the following Expression
(Xxe2x88x920.707)2+(Yxe2x88x920.5)2xe2x89xa60.0004xe2x80x83xe2x80x83(15) 
is satisfied.
Further, the present invention relates to a plastic optical fiber having a first core, a second core with a different refractive index from the first core, disposed concentrically on the outside of the first core, and a clad disposed concentrically on the outside of the second core, wherein:
a ratio X (r1/r2) of a first core radius r1, to a second core radius r2 and a ratio Y (n2/n1) of a refractive index difference n2 between the second core and the clad to a refractive index difference n1, between the first core and the clad are in a range where the following Expression
(Xxe2x88x920.775)2+(Yxe2x88x920.4)2xe2x89xa60.0004xe2x80x83xe2x80x83(16) 
is satisfied.
Further, the present invention relates to any optical fiber as set forth above, wherein:
the first core and the second core are each composed of one kind of polymer;
a compatibly blended layer of a polymer blend which consists of polymers constituting neighboring layers, respectively, is each formed between layers of the first core and the second core and between layers of the second core and the clad; and
a thickness of either of said compatibly blended layers is not greater than 10 xcexcm.
Further, the present invention relates to a plastic optical fiber having a first core, a second core with a different refractive index from the first core, disposed concentrically on the outside of the first core, and a clad disposed concentrically on the outside of the second core, wherein:
a ratio X (r1/r2) of a first core radius r1 to a second core radius r2 and a ratio Y (n2/n1) of a refractive index difference n2 between the second core and the clad to a refractive index difference n1 between the first core and the clad are in a range where the following Expressions
Yxe2x89xa7xe2x88x921.134X+1.0518xe2x80x83xe2x80x83(1) 
Xxe2x89xa6xe2x88x921.4842Y2+1.1097Y+0.7097xe2x80x83xe2x80x83(2) 
Yxe2x89xa60.7xe2x80x83xe2x80x83(3) 
Xxe2x89xa70.5xe2x80x83xe2x80x83(4) 
are satisfied;
the first core and the second core are each composed one kind of polymer;
a compatibly blended layer of a polymer blend which consists of polymers constituting neighboring layers, respectively, is each formed between layers of the first core and the second core and between layers of the second core and the clad; and
a thickness of either of said compatibly blended layers is not greater than 10 xcexcm.
Further, the present invention relates to any plastic optical fiber as set forth above, wherein a compatibly blended layer of a polymer blend which consists of polymers constituting neighboring layers, respectively, is not formed, at least, either between layers of the first core and the second core or between layers of the second core and the clad.
Further, the present invention relates to any plastic optical fiber as set forth above, wherein a xe2x88x923 dB bandwidth measured at a fiber length of 50 m under the condition of all mode excitation is not less than 400 MHz.
Further, the present invention relates to a plastic optical fiber cable comprising any plastic optical fiber as set forth above, the outer surface of which is coated with a resin.
Further, the present invention relates to an optical transmission device, comprising any plastic optical fiber or plastic cable as set forth above, a light emitting element with an excitation NA of not less than 0.3, disposed at one end thereof, and an optical receiver disposed at the other end thereof.
Further, the present invention relates to an optical transmission device, comprising:
a plastic optical fiber having a first core, a second core with a different refractive index from the first core, disposed concentrically on the outside of the first core, and a clad disposed concentrically on the outside of the second core, wherein: a ratio X (r1/r2) of a first core radius r1, to a second core radius r2 and a ratio Y (n2/n1) of a refractive index difference n2 between the second core and the clad to a refractive index difference n2, between the first core and the clad are in a range where the following Expressions
Yxe2x89xa7xe2x88x921.134X+1.0518xe2x80x83xe2x80x83(1) 
Xxe2x89xa6xe2x88x921.4842Y2+1.1097Y+0.7097xe2x80x83xe2x80x83(2) 
Yxe2x89xa60.7xe2x80x83xe2x80x83(3) 
Xxe2x89xa70.5xe2x80x83xe2x80x83(4) 
are satisfied,
a light emitting element with an excitation NA of not less than 0.3, disposed at one end of the plastic optical fiber; and
an optical receiver disposed at the other end of the plastic optical fiber.
According to the present invention, there can be provided a POF with a double-layered core, having a high transmission power and a wide bandwidth. Further, there can be provided an optical transmission device capable to transmit signals at high speed.